Modern day aircraft are now of such size and/or fly at such airspeeds that power boost through the use of hydraulic actuator assemblies is required so the pilot is able to move the control surfaces of the aircraft.
Such control surface hydraulic actuator assemblies usually include a hydraulic actuator, a control valve to control the movement of the hydraulic actuator, means which feed commanded inputs to the control valve and means which feed back the actuator's position to the control valve so the hydraulic actuator is moved to and positioned in the commanded location. Unfortunately, control valves in some instances have a tendency to jam. This causes the valve spool or slide therein which normally switches the pressurized hydraulic fluid fed to the hydraulic actuator to move it, to remain in an undesirable fixed position. If unchecked, a spool jammed in a fluid transmitting position causes the hydraulic actuator and the control surface connected thereto to "runaway" to one or the other extremes of travel thereof.
Competent aircraft design and federal regulations dictate that unless a control valve jam can be shown to be extremely improbable, a jam must be protected against or the effect thereof alleviated if the jam can cause a flight control surface to runaway to an adverse position. Since the jam is probable, as aforesaid, four types of prior art valves have been developed to solve the runaway control problem.
One of these prior art control valves has dual concentric slides, each slide is designed to provide one-half of the required hydraulic flow to the hydraulic actuator and both slides operate in sequence. If a concentric slide jams near neutral approximately half the flow is available to feed the hydraulic actuator for the control of the control surface. If, on the other hand, a concentric slide jams hardover; that is, in a position to port maximum flow to the actuator to move it in one direction, both actuator cylinder pressures are ported to return and the actuator is in a bypassed condition. Not only is the dual concentric slide valve disadvantageous as causing unnecessary hydraulic flow when jammed in anything but a null position, it can jam near a null position and the jam is not detectable since up to one-half the flow is still available to make the hydraulic actuator operate normally. Since most flight conditions do not require anywhere near the full flow which would cause the dual concentric valves failure to be obvious, the jam may go undetected until the other slide therein jams. If it jams in the same direction as the first jammed slide there is a possibility of control surface runaway to a far extreme. Therefore, multiple control surfaces must be use to assure that in the event of a control surface runaway, sufficient authority is available in the other control surfaces to retain control of the aircraft.
The second prior art valve has a flow controlling slide which operates in conjunction with a caged outer sleeve. When the slide jams, the outer sleeve moves and bypasses the actuator cylinder ports.
The third prior art valve has a caged spring cartridge in the valve input drive. When the valve jams, the spring cartridge is overridden, tripping an electrical switch which, as long as electric power is available, energizes a solenoid operated cylinder bypass valve. It should be obvious that this third type prior art control valve which utilizes electric sensing means can lose its jam protection when an electronic failure such as a failed switch or the loss of the electrical system occurs. For this reason the electronic system must be highly redundant so the requirement of extreme improbability of a runaway flight control surface is met.
The fourth type is a split tandem valve as disclosed in U.S. Pat. No. 3,757,817, which has two slides in tandem attached to a driving member through caged spring cartridges. If one of the slides jams, the other is moved in the opposite direction by the feedback means thereby preventing a control surface runaway by placing the controlled tandem hydraulic cylinder in hydraulic lock.